In an optical module that adds and drops wavelength division multiplexing (WDM) light, there is a part formed by integrating a variable optical amplifier (VOA) that adjusts the intensity of signal light with a tunable optical filter (TOF) that adjusts a transmission center frequency of the signal light. In an optical module, by adjusting an angle of a single unit of two dimensional micro electro mechanical systems (MEMS) mirror by using two axes and by adjusting a first axis, for example, a transmission center frequency of the TOF is adjusted, whereas, by adjusting a second axis, for example, an amount of attenuation of the VOA is adjusted. Consequently, it is possible to improve the optical signal noise ratio (OSNR) while adjusting the amount of attenuation of the signal light.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-8562
Patent Document 2: Japanese Laid-open Patent Publication No. 2009-47917
Patent Document 3: U.S. Patent Application Publication No. 2004/0141756
Patent Document 4: U.S. Patent Application Publication No. 2008/0085119
However, with the MEMS mirror functioning as an optical module, the sensitivity is high due to covering a wide range of frequency band and thus a setting error of an angle is present due to product variations or the like. In the optical module, when the amount of attenuation of the VOA is changed, the second axis of the MEMS mirror needs to be adjusted; however, if a setting error is present, a transmission center frequency of the TOF may possibly be deviated. This situation is remarkably exhibited in high rate signal light having a large spectral width.
Thus, if a transmission center frequency of the TOF is deviated in accordance with a change in the amount of attenuation, the optical module adjusts the first axis of the MEMS mirror so as to eliminate the deviation of the transmission center frequency by using feedback control. However, time is needed for the optical module to eliminate the deviation of the transmission center frequency caused by a change in the amount of attenuation and thus the quality of transmission is degraded during this period of time.